(Adapted from applicant's abstract) We propose to conduct a prospective cohort study of preventable risk factors for asthma among participants in the Nurses' Health Study II (116,678 female nurses, aged 25-42 in 1989). We will begin by examining the dietary hypo-theses that high intake of antioxidants (e.g., vitamin C, vitamin E, and B-carotene), magnesium, potassium and n-3 polyunsaturated fatty acids decreases risk of asthma, whereas high intake of sodium and specific fatty acids (e.g., linoleic acid) increases risk. Dietary factors were measured in 1991 using a semiquantitative food frequency questionnaire that has been extensively validated. Incident cases of asthma will be identified by mailed question-naire, and then confirmed using a supplementary questionnaire and selective review of medical records. During four years of follow-up (1991-1995), we expect approximately 1,500 cases of confirmed incident asthma; this will provide a power of over 99 percent to detect a trend across quintiles of dietary intake. The rising incidence of asthma and its high societal cost make asthma prevention an important public health goal. We believe that this study, which is very cost effective since it makes use of an existing cohort, will provide information which could have direct clinical application to reduce risk of asthma.